1. Field of Invention
This invention relates to improvements in heat transfer from thermal inkjet printing devices. More particularly, this invention is directed to methods and apparatus for positioning a heat sink against a thermal inkjet printhead substrate to increase the efficiency of heat dissipation from the substrate.
2. Description of Related Art
Thermal inkjet printers generally include a thermal printhead for ejecting ink onto a recording medium, such as, for example, paper. The thermal printhead has a plurality of ink channels formed in a substrate of the printhead. Each channel has a resistor to heat and selectively vaporize ink near the nozzle of that capillary filled ink channel. The vaporized ink forms a bubble that temporarily expels an ink droplet and propels it toward the paper. Carriage type inkjet printers include a carriage which moves the printhead across the face of the paper.
Thermal inkjet printhead substrates become heated as a result of the process which is used to vaporize the ink. Excess heat is generally allowed to slowly dissipate into the surrounding environment. The amount of space available within the printer casing is typically limited. Thus, it has proven difficult to provide efficient methods and devices that efficiently remove heat from the printhead substrate to the surrounding air. This has become more critical in view of the increased emphasis on reduced printer case footprints and higher through-put (pages per minute) abilities, particularly in carriage-type inkjet printers.
This invention provides methods and apparatus that provide an effective heat transfer pathway to remove heat from a thermal inkjet printhead substrate.
This invention separately provides a heat sink positioned against the back face of the substrate that forms an effective heat transfer pathway.
According to an exemplary embodiment of the method and apparatus of this invention, a heat sink, preferably of the finned plate type, is adapted to provide a heat transfer pathway to remove heat from the rear face of a printhead substrate. The heat sink is preferably located between the printhead substrate and the printer carriage. The heat sink may, in one embodiment, be biased to float substantially normal to the rear face of a printhead substrate using a biasing device, such as a spring or springs, or a spring clip or spring clips or other clamps or clamping mechanisms. The springs, which are compressed, tend to urge the face of the heat sink against the back face of the printhead, forming an interface between the two faces.
The springs produce an amount of force effective to press the heat sink against the substrate. Additional force is provided using the carriage arm, as its radius acts as a moment arm to assist in pressing the two faces together, which makes it easier for an individual to lock the printhead in position against the force of the springs. A low thermal heat conductivity contact resistance and an improved thermal resistance to heat dissipation between the printhead substrate and the heat sink are provided by the high pressure interface resulting from the use of the springs and the radius of the carriage arm.
An additional layer of heat conductive material is preferably provided at the interface of the heat sink and the printhead substrate to reduce the ability of air pockets to form between the heat sink and the printhead substrate. The additional layer is preferably a gap filler material such as a heat conductive polymer, an interface adhesive, liquid heat sinks, fabric, thermal grease or other thermal interface material or materials material that are effective to fill rough areas of surface on the printhead substrate and/or on the heat sink contact areas. Without the additional layer, such rough contacting surfaces may produce air pockets which can reduce the efficiency of heat transfer out of the printhead substrate and into the heat sink.